1. Field of the Invention
This invention relates to lens testing apparatus by which the focus of a lens can be set to sharply image a distant object on a select focal or reference plane and by which the focal length of a lens can be determined.
2. Description of the Prior Art
In the practice of the optical arts, it is of such fundamental importance to be able to measure the focal length of a lens and to be able to set the focus of a lens for best imagery in a given reference plane that those skilled in the art have developed a number of apparatus and methods for these purposes.
One of the most-known and accurate methods, which is particularly suitable for measuring medium focal length lenses, employs the nodal slide. The essential part of the nodal slide is the provision for moving the lens system longitudinally with respect to a vertical axis of rotation. The vertical axis is mounted so that it may be positioned longitudinally with respect to a collimator of appropriate size. Usually the object for the collimator is a very small point source set at the focal point of the collimator. In use, a magnifier or microscope is set up approximately at the lens focal plane. The lens under test is then moved backward and forward along the nodal slide until rotation of the nodal slide through a small angle produces no sidewise shift in the image seen through the microscope. A measure of the focal length is then the distance between the axis of rotation of the nodal slide and the appropriate focal point located with the magnifier or microscope. The focal length can also be specified with respect to the vertex of the rear surface (this distance is known as the "back focal length") or to any other convenient part of the lens. When the focal length is specified by the distance between the focal point and some convenient reference point on a lens mount, it is understood that this is the "flange focal distance". On the other hand when the focal length is specified as the distance between the focal point and the second nodal point of the optical system, the "effective focal length" has been specified. For a more detailed description of the nodal slide method and apparatus, reference may be had to standard textbooks in the optical field such as "Modern Optical Engineering", by Warren E. Smith, published by McGraw-Hill Book Co., New York, 1966.
Another basic method for locating the focal point of a lens employs auto-collimation. Here an illuminated target and a screen are placed side by side near the estimated focus of the lens under test, and a plane mirror is placed in front of the lens so as to reflect light from the target back into the lens. The target and screen are moved until the reflected image of the target is sharply imaged on the screen. Both screen and target then lie in the focal plane. For accurate work, an auto-collimating microscope produces excellent results. Again, "Modern Optical Engineering" by Smith may be referred to at pages 434 and 435 for a more detailed description of auto-collimation.
In photograhic cameras, a popular method for setting the focus of the lens for best imagery at the film plane of the camera employs a ground-glass plate. Here a suitable target is located at a reference distance forward of the camera, and the image formed by the lens on the ground glass plate is observed through the use of a microscope. The lens focus is adjusted until the image shows maximum quality as judged by the observer. However, in this method, the image-quality judgment is subjective and there can be significant variations among different observers and with the same observer at different times. Although this ground-glass technique is rather simple, the possibility for error because of observer subjectivity may make it undesirable in a particular photographic application.
All of the foregoing methods and apparatus are most suitable for use in laboratory settings where rapid determination of focal length or setting of lens focus is not required and also require well-trained observers to carry out the measurements. Because of these limitations, other apparatus have been developed which are more suitable for use in high volume production settings requiring both speed and accuracy. Examples of apparatus suitable for this purpose are disclosed and described in, for example, U.S. Pat. No. 3,323,417 issued to David S. Grey et al. on June 6, 1967 and entitled "Testing Apparatus for Optical Lenses" and U.S. Pat. No. 3,619,067 issued to Bradford Howland et al. on Nov. 9, 1971 and entitled "Method and Apparatus for Determining Optical Focal Distance". The apparatus described in these two patents operate to determine the axial distance from some convenient point in or on a lens to the minimum on-axis blur circle which is formed by the lens. The minimum blur size is indicative of the focus position at which the largest amount of energy from the lens passes through the minimum sized area when the lens is illuminated by a collimated source. It appears that these apparatus can be adapted to permit setting of lens focus for best imagery in a select reference plane.
U.S. Pat. No. 3,904,294 issued to Nathan Gold et al. on Sept. 9, 1975 and entitled "Automated Lens Testing Apparatus" discloses lens testing equipment by which acceptance testing of photographic objective lenses is determined by evaluating the optical performance at several representative locations in a reference plane as lens focus is adjusted to determine the focus setting at which the performance at each of the locations meets certain minimum standards as based on the contrast transfer efficiency of a target having a predetermined spatial frequency at each test location. With this apparatus, best focus setting can be determined or the lens can be rejected for failure to meet minimum standards.
Although the prior art discloses methods and apparatus appropriate to the tasks of focal length measurement and setting lens focus, it is evident that the need for such test apparatus in the optical arts is continual, and it is therefore a primary object of the present invention to provide such apparatus.
It is another object of the present invention to provide novel test apparatus and methods for setting lens focus and measuring lens focal length through the use of a novel combination of readily available components and steps.
Other objects of the invention in part will be obvious and in part will appear hereinafter. The invention accordingly comprises the methods and apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed description.